Interpretive Summary: Field studies were conducted in 2008 and 2009 in ID, OR,and WA potato growing regions to determine the effect of simulated glyphosate drift on potato during the application year and the crop growing the next year from the daughter tubers. Glyphosate was applied at five low doses to simulate drift, which corresponds to 1/100, 1/16, 1/8, 1/4, and 1/2 of the lowest recommended application rate in glyphosate resistant corn. Glyphosate was applied at four stages of potato growth; when potato plants were 10 to 15 cm tall (Early), or at stolon hooking (H), tuber initiation (TI),or during mid-bulking (MB). In general, the MB applications caused less visual foliar injury to the mother crop than earlier applications at ID or OR, and applications at the H timing at WA. Mother crop injury increased as glyphosate rate increased regardless of location, application timing, and rating date. U.S. No.1 and total tuber yields were usually related to the visual injury level observed on the mother crop. Although injury to the mother crop from glyphosate applied at the later MB stage lowest, the injury to plants growing from daughter tubers from that timing of application planted the following year was similar or greater than that of earlier timing treatments. MB daughter tuber emergence was less than 30 % of the nontreated control tuber emergence while emergence of daughter tubers from the other treatments was 60 to 95 %. As rate of glyphosate applied to the mother crop increased, daughter tuber emergence decreased. When MB daughter tubers did emerge, plants were chlorotic and stunted as if the plants had been directly sprayed with glyphosate. Regardless of whether the daughter tubers had visual defects or not, results the following year were the same. Implications are that if a mother seed crop encounters glyphosate during bulking, injury may not even be noticeable on the foliage or the tubers, however, emergence, vigor, and yield of the crop growing the following year from the daughter tubers could be greatly impacted.

Technical Abstract:
Field studies were conducted in 2008 and 2009 in Aberdeen, ID, Ontario, OR, and Paterson, WA to determine the effect of simulated glyphosate drift on ‘Ranger Russet’ potato during the application year and the crop growing the next year from the daughter tubers. Glyphosate was applied at 8.5, 54, 107, 215, and 423 g ae ha-1 which corresponds to 1/100, 1/16, 1/8, ¼, and 1/2 of the lowest recommended single application rate for glyphosate-resistant corn and sugar beet of 846 g ha–1. Glyphosate was applied when potato plants were 10 to 15 cm tall (Early), or at stolon hooking (H), tuber initiation (TI), or during mid-bulking (MB). In general, the MB applications caused less visual foliar injury to the mother crop than earlier applications at ID or OR, and H applications at WA. Mother crop injury increased as glyphosate rate increased regardless of location, application timing, and rating date. U.S. No.1 and total tuber yields were usually related to the injury level resulting from glyphosate application timings and rates. Although injury to the mother crop from glyphosate applied at MB usually was the lowest compared to injury from other application timings, when daughter tubers from that timing were planted the following year, emergence, plant vigor, and yield was most detrimentally impacted compared with that of daughter tubers from other timing treatments. MB daughter tuber emergence was less than 30 % of the nontreated control tuber emergence while emergence of daughter tubers from the other treatments was 60 to 95 %. As rate of glyphosate applied to the mother crop increased, daughter tuber emergence decreased. When MB daughter tubers did emerge, plants were chlorotic and stunted as if the plants had been directly sprayed with glyphosate. Regardless of whether the daughter tubers had defects or not, results the following year were the same. Implications are that if a mother seed crop encounters glyphosate during bulking, injury may not even be noticeable on the foliage or the tubers, however, emergence, vigor, and yield of the crop growing the following year from the daughter tubers could be greatly impacted.